Gallium Nitride (GaN) based switches, and other similar High Electron Mobility Transistors (HEMTs) based on heterojunctions, provide high voltage support, low drain-to-source on resistance, low gate-drive charge requirements, and fast switching. As a result of these characteristics, GaN-based switches are increasingly being used in applications that require high efficiency and high-frequency support, including, notably, switching power converters. However, some GaN-based switches have unique gate-drive requirements, as compared with conventional metal-oxide semiconductor field-effect transistors (MOSFETs) and bipolar junction transistors (BJTs), and typically require complex gate-drive circuitry.
A GaN-based switch in its native state is a normally-on (depletion-mode) device. Such a device conducts current from its drain to its source when no voltage is applied to its gate, relative to its source, and requires application of a negative voltage to its gate to force the device into a non-conducting (blocking) state. Such normally-on behavior is unsuitable for most applications. Hence, modifications to GaN-based switches have been developed so as to convert them into normally-off (enhancement-mode) devices. For example, a p-doped GaN layer introduced between the gate metal and the heterostructure of a GaN-based switch has the effect of raising the switch's turn-on/off voltage threshold to a positive value, thereby providing a normally-off device. Enhancement-mode switches based on such a gate structure are known as Gate Injection Transistors (GITs).
GaN-based GITs have a relatively low threshold voltage for switching between their conducting (on) and blocking (off) states. This threshold voltage is typically in the range of 1.2 to 3.5V, which is significantly lower than corresponding thresholds, e.g., 5V, for other power MOSFETs. Additionally, HEMTs, including GaN-based GITs, have low gate-to-source and gate-to-drain capacitances, which are notably smaller than corresponding capacitances in other power MOSFETs. While the low threshold voltage and low gate capacitance of a GaN-based GIT advantageously provide fast switching speeds and low gate charge requirements, these characteristics also make a GaN-based GIT susceptible to being undesirably turned on due to voltage perturbations at the gate of the GIT during intervals when the GIT is intended to be held in its non-conducting (blocking) state. For example, noise at the gate could cause its voltage to rise above the GIT's threshold voltage, though the gate is intended to be held at a low voltage. Such noise may occur during operational intervals when the GIT is intended to be held in its non-conducting state, and during start-up intervals during which the gate may not yet be provided with a driven control signal. Additionally, the gate voltage may be susceptible to ringing after the control voltage is transitioned from a high (turn-on) voltage level to a low (turn-off) voltage level. The voltage level of the ringing can exceed the GIT's threshold voltage, thereby unintentionally turning on the GIT.
The above problems are conventionally addressed using complex circuitry customized for driving GaN-based GITs or similar enhancement-mode HEMTs. Such circuitry typically drives a negative voltage onto the gate to turn off the GIT, thereby providing significant margin between the driven gate voltage and the turn-on threshold voltage of the GIT. This margin allows the GaN-based GIT to be reliably held in its non-conducting (blocking) state. A resistor-capacitor (RC) circuit is often included in the driving circuitry, so as to provide high current when the GaN-based GIT is initially transitioned to a conducting state. Lower current is provided subsequently to maintain the conducting state of the GIT. The RC circuit additionally has the effect of applying a relatively high magnitude of the negative voltage when the GaN-based GIT is transitioned off, and this negative voltage dissipates towards zero as the off interval proceeds. Typical driving circuitry, as described above, includes at least two and as many as four driver switches, each of which must be controlled, and provides three or four voltage levels to the gate.
The typical GIT driver circuitry described above has many problems. First, the negative voltage provided at the gate during the turn-off interval leads to a large required voltage swing when the GIT is transitioned to its conducting state, thereby slowing the transition and the potential switching speed of the GIT. Second, the RC-based dissipation means that the level of the negative voltage will vary according to the switching duty cycle, thereby leading to transition times that are inconsistent, which complicates the use and control of the GIT. Third, while the negative voltage described above reliably holds the GIT off during steady-state operation, spurious non-zero voltage during an initial start-up interval, before the negative voltage is driven to the gate, may undesirably turn on the GIT. Fourth, the negative voltage adds an offset to the effective reverse body diode voltage, thereby increasing the threshold voltage of the effective reverse body diode and increasing associated losses. Lastly, the driver circuitry is quite complex, and requires fairly complex control of switches within the driver circuit itself.